The technology in which a sputtering vaporization source, such as a planar magnetron, is used and a metal target, e.g., an aluminum target is sputtered in an atmosphere of a reaction gas, e.g., oxygen (O2), is a known technology for evaporating a film of a metal compound, e.g., alumina (Al2O3) at a high speed. It is always a technical problem to control such a reactive sputtering process and to control the component ratio of a film while the speed of evaporation or film formation is maximized.
The reactive sputtering is a very flexible coating technology, and allows various compound materials, e.g., Al2O3, to be produced by the use of an aluminum target and a reaction gas, O2.
However, a significant problem has been included therein. More specifically, when the partial pressure of a reaction gas, e.g., O2, reaches a level suitable for forming a film of a metal compound having a predetermined component ratio on a surface of a substrate, formation of the same metal compound also starts on a surface of the metal target. As a result of the formation of the metal compound on the surface of the target, the amount of metal which vaporizes from the target is reduced, and a film formation speed is lowered. In this manner, the amount of vaporized metal capable of reacting with the reaction gas in a chamber is reduced and, thereby, the partial pressure of the reaction gas in the chamber is increased.
Consequently, as shown in FIG. 1, the characteristic of the relationship between the change in the amount of the reaction gas introduced into the chamber and the reaction gas partial pressure in the process of increase of the reaction gas flow rate is different from that in the process of reduction while an electric power of the sputtering is kept constant, and in the characteristics exhibited, the reaction partial pressure jumps discontinuously.
At this time, in the condition in which the partial pressure is low as indicated by A shown in the drawing, although the film formation speed is high, generally, a coating containing excess metal tends to be formed. Under this condition, the sputtering target is kept in the state in which the metal is exposed at the surface, and this film formation condition is referred to as “a metal mode”. Conversely, in the condition in which the partial pressure is high as indicated by B shown in the drawing, a compound coating made from the metal and the reaction gas is formed on the substrate. Under this condition, the compound is also formed on the surface of the sputtering target, and the film formation speed is significantly reduced compared with that in the metal mode. This film formation condition is referred to as “a poisoning (pollution) mode”.
As described above, transition between the two modes, the metal and the poisoning modes, occurs rapidly, and as a result, it may be difficult that such a reactive sputtering and the formation of compound at a high rate become mutually compatible.
In order to overcome the above-described instability of the reactive sputtering, the following various methods for controlling the film formation have been proposed.
1. A method in which the reaction gas partial pressure in a chamber is monitored, and the amount of introduction of the reaction gas is adjusted (refer to Japanese Unexamined Patent Application Publication No. 4-136165).
2. A method in which the plasma emission intensity is monitored, and the flow of the reaction gas is adjusted (refer to Japanese Unexamined Patent Application Publication No. 7-94045, Japanese Unexamined Patent Application Publication No. 62-211378, and Japanese Unexamined Patent Application Publication No. 62-180070).3. A method in which the sputtering voltage is monitored, and the flow of the reaction gas is adjusted to keep the sputtering voltage constant (refer to Japanese Unexamined Patent Application Publication No. 4-325680).4. A method in which the voltage of the sputtering power source is controlled at constant.
With respect to a technology proposed as the above-described first method, in a reactive sputtering of, for example, aluminum and oxygen, the partial pressure of O2 in a chamber is monitored, and when a reduction of the pressure of oxygen is detected, the flow of O2 is increased, so that the reactive sputtering of aluminum is controlled, and compounds, such as Al2O3, are formed.
In this method, an oxygen flow valve is controlled in order to keep an oxygen gas partial pressure constant and, thereby, the sputtering process is controlled.
By the use of this control, the above-described discontinuous characteristic between the reaction gas and the partial pressure can be stabilized, and as shown in FIG. 2, stabilization can be achieved due to the characteristic having a reverse slope between the reaction gas flow rate and the gas partial pressure. As a result, it is believed that the compound can be formed at a high speed. This film formation region between the two modes, the metal and the poisoning modes, is referred to as “a transition mode”.
The above-described second method is a method in which plasma emission spectrum is monitored, and the flow of the reaction gas is controlled. In this method, the plasma emission of the glow discharge causing sputtering is spectroscopically analyzed, and the spectral intensity thereof is brought into correspondence with the amount of metal sputtering-vaporized and the partial pressure of the reaction gas, so that the control is performed.
In the most frequently used technique of this method, the emission of the wavelength band intrinsic to a metal element in the plasma emission is used.
A metal is vaporized by sputtering with only an inert gas without introducing any reaction gas, the emission intensity of the wavelength band intrinsic to the metal element is monitored, and this emission intensity is referred to as Im (Max). When a reaction gas is introduced and the reactive sputtering is effected, the formation of a compound on a target is started, and a reduction of the amount of sputtering vaporization is started. The amount of metal element present in the glow discharge is decreased with this reduction of the amount of sputtering vaporization and, thereby, the emission intensity Im of the wavelength band intrinsic to the metal element is reduced.
In this method, the amount of introduction of the reactive gas is controlled in order that this Im becomes a predetermined proportion relative to the Im (Max), so that the film formation in the transition mode is realized, and many examples of success have been reported.
The above-described third method is a method in which the sputtering voltage applied to a sputtering cathode is monitored, and is set at a predetermined value by controlling the reaction gas flow rate.
As shown in FIG. 3, this takes advantage of the change of the voltage of the sputtering cathode depending on the film formation mode. For example, in the reactive sputtering through the combination of aluminum and oxygen, the voltage of the sputtering cathode takes on a value close to a high 400 V in the metal mode, the voltage of the sputtering cathode takes on a value in the neighborhood of a low 320 V in the poisoning mode, and a value in between them is taken on in the transition mode. Since the film formation mode can be determined by measuring the voltage of the sputtering cathode through the use of this characteristic, for example, the above-described Japanese Unexamined Patent Application Publication No. 4-325680 proposes to control the reaction gas flow rate in order that the voltage of the sputtering cathode becomes a desired value in the sputtering method by the use of two sputtering cathodes, referred to as a dual magnetron sputtering method.
With respect to the above-described fourth method in which the sputtering voltage is controlled at a constant value, the control of reactive sputtering of an AIN film by the use of an aluminum target and an Ar/N2 sputtering gas mixture is disclosed in the Journal of Vacuum Science Technology, A20 (3), pages P376 to 378, March 1982, “Voltage Controlled, Reactive Planar Magnetron Sputtering of AlN Thin Films” by McMahon, Affinito, and Parson, and is studied in the Journal of Vacuum Science Technology, A2 (3), pages P1275 to 1284, July to September 1984, “Mechanisms of Voltage Controlled, Reactive Planar Magnetron Sputtering of Al in Ar/N2 and Ar/O2 Atmospheres” by Affinito and Parsons.
In these two papers, the control of gas supply, power, current, and voltage for forming an AlN thin film having a predetermined component ratio by the reactive sputtering of aluminum in an argon/nitrogen mixed gas is discussed.
In the above-described papers, authors concluded that the constant gas flow rate and the control of the voltage were most suitable for reactive planar magnetron sputtering of aluminum.
The inventors of the present invention conducted experiments by the use of an aluminum target and oxygen serving as a reaction gas, and also verified that the method in which a sputtering power source was operated in a voltage control mode was effective at controlling the discharge of sputtering to the transition mode.
FIG. 4 shows the voltage-current characteristic of a sputtering cathode in the case where this voltage control was used. It is shown that when the sputtering power source was subjected to the current or power control, the characteristic (broken line) exhibited a discontinuous jump of the discharge mode and stable discharge was impossible in the transition mode, whereas when the voltage control was conducted (solid line), discontinuity of the discharge mode did not occur and the discharge was able to be stably operated in the transition mode.
In the above-described first method, the response time of the system is slow. That is because the partial pressure of the reaction gas sampled from a sputtering chamber must be analyzed, the flow of the reaction gas into the sputtering chamber must be adjusted based on the results thereof, and the flow must be diffused into the inside of the chamber so as to effect a desired change in the reaction gas partial pressure.
If the apparatus is upsized, it is difficult to ensure stabilization by this control method due to the problem of this response time.
In the second method, the means used for the control is not the reaction gas partial pressure, that is, indirect information, but the emission intensity directly correlated to the amount of vaporization of the metal. Therefore, the controllability is believed to be better than that in the above-described technique through the use of the reaction gas partial pressure.
However, according to the experiments conducted by the inventors of the present invention, stable control was not able to be achieved, although the method was applied to the film formation in a large treatment volume. The reason is believed that even when the amount of introduction of the gas was changed, the diffusion in the inside of the chamber took a time, and the time elapsed before the occurrence of change in the sputtering method was increased.
In the above-described third method, according to the experiments conducted by the inventors of the present invention, it was difficult to stably fix the film formation mode to the transition mode. The reason may be the same as that in the above-described second method.
The fourth method was effective with respect to even an apparatus having a large chamber. The reason is believed that in the stabilization of the discharge mode by the voltage control of the sputtering power source, since the stabilization of the power source voltage is performed by a constant-voltage control circuit in the inside of the sputtering power source, a factor of time delay, such as the diffusion of gas in the above-described system for controlling the flow rate of the reaction gas, is not included.
However, in the experiments on the fourth method, conducted by the inventors of the present invention, it was made clear that the following problems also occurred in this control system (the voltage control of the sputtering power source).
That is, in the style of this control system, the film formation mode is determined by a voltage command value to the sputtering power source, and the voltage-current characteristic is influenced by the other process conditions, such as the amount of introduction of the reaction gas and the amount of introduction of the discharge gas. For example, FIG. 5 shows the discharge characteristics in the case where the amount of introduction of the reaction gas is changed in the alumina film formation. The reactive sputtering can be stably controlled by the voltage control of the sputtering power source. However, it is observed that the curve of the voltage-current characteristic tends to shift toward higher sides of both the voltage and the current as the amount of introduction of the reaction gas is increased. Consequently, even when the voltage of the sputtering power source is set at a constant value, there is a problem of shift from a predetermined mode due to the above-described change in the characteristic values.
For example, in the case where the sputtering voltage is controlled at a voltage Vs in an example shown in FIG. 5, when the reaction gas flow rate is low (A), it is possible to control to the transition mode, whereas when the reaction gas flow rate is high (B), the film formation is performed in the poisoning mode.
Similar problems occur due to the amount of a residual gas in the chamber. In particular, in the case where a plurality of substrates are mounted, and film formation is performed by raising the temperature of the substrates, even when the amount of introduction of the reaction gas is the same, the shift (C) of the voltage-current characteristic occurs under the influence of the gas release from the substrates, and a constant film formation mode cannot be kept only by keeping the sputtering voltage constant.
Furthermore, in the case where the type of film to be formed is an insulating coating, such as alumina, an increase (D) of the voltage occurs with a lapse of the film formation time under the influence of the insulating film deposited on not only the substrate but also the chamber. Consequently, it is difficult to keep the film formation mode only by the voltage control in this case as well.